NIH Human Stem Cell Facility is a group at the Bethesda campus that is focused on characterizing the conditions for growing human embryonic stem cells. Because hES cells give access to early developmental events and can generate many terminally differentiated cell types, they may form the basis of a powerful new technology. Despite this promise, technical limitations may blunt the wide use of human ES cells. The most fundamental concern would be caused by genetic instability. The NIH-HSCF has routinely grown hES cells from August 2003. In the following year, most of the hES cells on the NIH registry were obtained, expanded in culture and analyzed by karyotype and FACS analysis. In the period to 7/2004, the central achievement of NIH-HSCF was to show that sub-clones of a hES cell line had a stable genome using a high-resolution genome scanning method. This result provides the most convincing evidence to date that at least one hES cell line can be grown for long periods without genetic change. In the most recent reporting period to 7/05, the major focus has been to use the skills of the staff in the facility to develop general standards to assess many hES cell lines. This work is an international collaboration organized through the International Stem Cell Consortium. The ISCC is sponsored by the International Stem Cell Forum, a grouping of research agencies in several countries including Australia, Canada, Israel, Sweden, the United Kingdom and the United States. The ISCC has developed a set of standard antibodies and a gene array to probe the differentiated state of hES cells. The ISCC is also promoting a careful analysis of the safety of hES cell lines. The NIH-HSCF is one of the few sites contributing multiple hES cells from different providers grown under standard conditions. As soon as the data is compiled it will be presented in a publicly accessible web site.